Sulfur-coated urea (SCU) slow release fertilizers are known. In the customary process for the production of sulfur-coated urea, granular urea of nominal size range, 1.7-2.9 mm, which has been preheated in a fluid bed heater to 160.degree. F. to 180.degree. F., is introduced into the front end of a rotating horizontal cylindrical drum, nominally 12 feet in length and 5 1/2 feet in diameter. Lifting flights, or longitudinal ledges, which are fastened to the inside wall of the drum and evenly spaced around its circumference, lift and cascade the urea granules as the drum rotates. As the cascading granules pass through the drum, molten (290.degree. F.) sulfur is sprayed onto the urea granules from a series of nozzles uniformly positioned within the length of the drum. When a droplet of molten sulfur contacts a granule, it quickly solidifies; and a continuous coating of sulfur is formed on a urea granule when a sufficient number of molten sulfur droplets have made contact with it. In this randomized coating process the granules are coated to an average target thickness of 40 microns (.mu.) or about 13%-14% by weight sulfur-coating on the urea. However, various random proportions of the granules, which are discharging from the drum, have thin (&lt;30.mu.), medium (30.mu.-50.mu.) and thick (&gt;50.mu.) sulfur-coating thickness.
Because of the inherent brittleness of the crystalline solid sulfur-coating which forms on the granule, and the thin, or even noncontinuous coating on many of the granules, it is customary that some type of secondary outer coating or sealant be spray applied onto the sulfur-coating surface. Usually this is done in a second horizontal rotating drum in series with the sulfur-coating drum. Historically, this sealant has been either a polymeric hydrocarbon, petroleum-based wax, or a combination of high viscosity polymeric paraffinic oil plus polyethylene, which is spray applied as a hot melt liquid onto the hot, but solid sulfur-coating surface. Since the sealant melt will not solidify at the 160.degree.-180.degree. F temperature of the sulfur-coated urea granules onto which it is applied, the liquid sealant distributes relatively uniformly onto all sulfur-coated granules by flowing, or transferring, from one granule to the next as they cascade through the rotating secondary sealant coating drum. These sealant coated sulfur-coated urea granules pass through a fluid bed cooler, after they are discharged from the sealant drum, wherein the sealant solidifies to a firm, but somewhat tacky polymeric coating.
The characteristic tackiness, which is common to these types of sealants, is offset by the application of a powder material, often referred to as a conditioner, such as talc, clay, silica, or diatomaceous earth. This conditioner application usually is applied to the cooled, polymeric coated sulfur-coated urea (PCSCU) in a third rotary drum in series with the sulfur-coating and sealant coating drums. Products of the above type are described in U.S. Pat. Nos. 3,295,950; 3,342,577; 3,877,415; 3,903,333; 3,991,225 and 5,219,465.
Although these sulfur-coated urea products generally have good slow release characteristics as manufactured, the impact and abrasion resistance of the coating is low. Mechanical degradation of the coating occurs as a consequence of the handling, transportation, blending and application operations, which results in significant deterioration of slow release properties. Therefore, improved products still are desired.